Amplifying system



" a0. bridge; stunning; typical group:

. Patented Nov. 17, 1931 ALBERT w. norm, or

comment, mwroax, assrenoa are amt courm, a com'ona'rron or new roananaemic mums srs rnrr age medium 24,

The present invention relates to amplifying systems containinglightsensitive devices and more particularly to circuits that employ, incombination, thermionic amplifiers an photo-electric tubes.

In view of the fact that the magnitude of the photographic currentderived from the commercial forms of light sensitiveapparatus whenactivated by sources oflight of .10 ordinaryluminosity is somewhatmeagre, usually of the order of micro-amperes, it is necessary to emploone or more stages of amplie fication in or er to obtain an outputcurrent of practical amount, i. e.,, suificient to actuate relays orother circuit control devices. This amplification conveniently may beprovided by one or more three-electrode thermionic. devices connected incascade with thephotoelectric cellg'a': tube.

An object of my invention is to provide circuit arrangements of thischaracter which will offer a high degree of sensitivity to changesof thecurrent. produced in the photoelectric tube in response to variations ofa light source. Another obj ect is to provide circuits which will besimple in both construction and operation, also which may be energizedeifectively from a source of alternating,- and under some conditions, ofdirect current.

so A still further object is to provide a light sensitive circuitarrangement in which under certain adjustments the output current varieslinearly with respect to the hoto-currents:

The novel features which I believe to be characteristic of my. inventionare set forth with particularity infthe appended clalms.

The invention itself, however, both as to'its organization and method ofoperation will best be understood by reference, to the followlo mg-description taken in'connection the accompanying drawings.

Figure-1 shows a bridge circuit organiza- "tion embodyingsome of. thefeatures of my.

invention including an amplifier exemplified in the form of agas-*filled'thermionic device having an arc-like characteristic and aphmto-electricf tube Figure 2 shows'the relation between 1 the anode an .ar t a g balanced" voltages under conditions of a maxi. semi m. 373,230..

of'three curves useful, in explaininga convement method ofcontrolling-the gaseous type of amplifier; Figure 4 depicts. therelation between the amplifier current and the illumination of thephoto-electric" cell of Fig. 1 durin changes in-the degreeofillumination an certain conditions of circuit adjustment while Figure5 graphically typifies theelectrical effects introduced into the circuitof Fig. 1 by various adjustments-0t the condenser.

Referring to Fi thermionic amplifier .of the grid-controlled arc discharetype. A device of this charac-- 'ter is descri din articles entitledHot cathode thyr atrons by A. W. Hull in the General Electric Review,Vol. 32, No. 2, A ril 1929, pages 213-223, and'Vol. 32, No. 7, uly 1929,

1, numeral 1 indicates a I pages 390-399. A Thy'ratron tube is charac-.

terlzed by large power output which is controllable by an exceedinglysmall amount of grid energy of the order of a fraction of a micro-watt.A device of this character usu allYcontains a three-electrodearrangement immersed in an ionizable medium, e. g. mercury vaporor.argon at a pressure between 1-50 microns of. mercu support anarc-like dls'charge at the impressed voltages. The electrodes maycomprise a thermionic cathode 2, an anode or plate member 4 and a gridor electrostatic control member 3 interposed, therebetween, allof theelectrodes being energized preferably b alternating'current irom amulti-tap 'secon ary 0011 of a transformer 5. The prisufliciently'highto through the tube as an arc whose starting.

only may be controlled by the grid but after starting, the current flowcan be stop p'id' .only' by'removing the-anode voltage. e gration ofsuch a device consists in perically removing the anode voltage to stopthe dischargrliscan -in controlling-the ingfof the v barge by the gnd;the latter v thus the average current of the fare. The grid effectivelyis connected to one tive member.

terminal of the transformer secondary through a variable condenser 7 andthe anode is connected to the other terminal through the load, thefilament current being taken from the transformer by means ofintermediate taps 8, 9 exemplified as being in a symmetrical position.There is a light-sensitive member 10 shown in the form of an ordinaryphoto-electric cell connected between the grid and the terminal of thetransformer secondary to which the load is connected. As shown, thecathode of the photo-electric cell is connected to the grid while theanode is connected to the transformer. A source of light 11 which may beof a variable character such. as receding or approaching daylight isprovided to activate the light-sensi- The load which may constitute arelay for controlling external circuits or an additional amplifier isconnected between the plate and the secondary terminal cogimon to theanode of the photo-electric tu e.

While the final adjustment of the system for operation will depend onthe type of response desired in the load circuit, i. e. whethersensitivity is required or whether a linear proportionality between theamount of light impinging on the photo-electric tube and the outputcurrent is desideratum, it is generally desirable first to effect abalance of the electrical bridge. This step is most convenientlyaccomplished by varying the condenser 7 while the photo-electric tube ismaintained in the dark state and noting the response of the arcdischarge tube a: will be explained hereinafter.

It is apparent that the system constitutes an electrical bridge in whichtwo arms comprise the condenser and coil portion 12, while the otherpair of arms is made up of the capacitative path between the controlmember 3 and anode 4 in parallel with the path through thephoto-electric tube 10, also thecoil portion 13. The path through thehotoelectric tube in the darkened state is a so capacitative since theohmic resistance'is infinite or substantially so. For convenience indiscussing the matter of bridge balance or departures therefrom, theinter-electro capacitances have been shown on the dia 1am in Fig. 1 asdotted condensers C 0, e (actively in shunt to theanode-cathode path ofthe photo-electric tube and to the grid-plate path of the amplifier. Thefilament-grid capacitance is not shown inasmuch as its magnitude is.relatively small. Thefilament taps may be positioned on the secondarycoil so that when the capacities in the opposite arms are properlyadjusted by varying the condenser the bridge may be balanced, in whichcase the capacitance C of the condenser 7 exactly neutralizes orbalancesthe combined effect of capacitances C 0,. Hence, no alternatingcurrent charge is applied to the grid during thepositive portion of thealternating current cycle for well understood reasons. However, eventhough the grid is substantially not charged under these conditions, thearc discharge device will normally start for the reason that a definitenegative grid potential at all times greater than the critical voltageis necessary to prevent initiation of the arc. This is clearly shown inFig. 2 wherein curve a represents the voltage applied to the anode bythe transformer secondary and the broken line 6 indicates the criticalnegative voltage necessary to restrain the tube from operating. An arcis prevented only in-the event that the grid voltage line 0 does notintersect the critical voltage curve 6. The curve 0 is drawn for anexact balance between the capacitance C1 and (G2+C hence, it is astraight line parallel to the horizontal axis whose position dependsonly on the leakage currents between thegrid and other elements of thetube and upon electrons received from the cathode. Under theseconditionsthe grid assumes a floating potential of about half a voltnegative with respect to the cathode. It will be observed in Fig. 2 thatthe grid voltage line 0 intersects the grid voltage curve 5 hence thearc will start and current will flow for practically the entire positiveportion of the cycle as indicated 'by the cross-hatched area. In case ahigh vacuum tube were substituted for the arc discharge device, it isevident that the voltage assumed by the grid when the bridge is balancedmay be sufiiciently negative to prevent flow of substantial platecurrent throughout the entire positive portion of the cycle. During thenegative portion of the alternating current cycle, no current can floweither through the arc discharge device or the high vacuum device forwell understood reasons.

When the photo-electric tube is activated by light from the source 11 sothat the resistance of tube 10 is materially decreased, the system nolonger remains in a balanced condition as will be explained hereinafterand an alternating potential is impressed on the grid. In the event thatthis potential is more positive than the critical amount necessary tooperate the thermionic device or less negative than the amount necessaryto 'restrain the tube from operating, an arc is initiated or triggeredwhich thereafter and until the end of the positive half-cycle is nolonger controllable by the grid. This condition is readily noted in thediagrams of F ig. 3. an arc' is started only when the grid voltage 0intercepts the critical voltage curve 7) during the interval the anodeis positive. It is evident that the constants of the circuit, also theposition of the filament taps may be adjusted to give the desired typeof response by way of current through the tube to operate relays or anyother type of load circuit. The phase of that portion of the currentthrough capacitative path C C in order to ofi-set or balance the currentflowing to the grid throu l1 capacitance C is now changed when the poto-electric tube is activated due to the introduction. of resistanceinto the branch containing capacitance (1 giving a grid voltage curve ofthe form shown in diagrams A, B and C, Fig. 3. Hence, the

grid is subjected to phase control having two aspects of current and nocurrent respectively when considered with respect to the action of thephoto-electric tube under may be controlled in this manner.

of the half cycle.

conditions of light and no light. However, if the light is progressivelychanged, the phase relation is likewise progressively varied as will bepointed out hereinafter and the average current flowing through the tubeIt is also apparent that the change in phase of voltage applied to thegrid may also be controlled by the condenser 7 assuming a light'ofconstant brilliancy. Under conditionsof a variable light source orvariations of the capacitance of condenser 7, the outputclirrent can bechanged continuously from zero to full value. This isclearly shown inFig. 3 wherein diagrams A, B and C represent'three typical stages in thephase control method of determining the point in the alternating currentcycle at which the arc shall start. In diagramA, the grid voltage isnearly 180 out of phase with the anode voltage and the current startsnear the end of the cycle; in diagram B, the grid voltage is advanced inphase so that the current starts at the middle of the cycle,'while indiagram C the grid voltage is nearly 'in phase with the anode voltageand current starts nearly at the beginning and flows for practically thewhole The amount of current per cycle is represented by the shaded areaand the; average current is the integrated amount per unit of time. Itis obvious that the-average current is determined by the point ofintersection between the grid voltage'curve '0 and the critical voltagecurve 6 which in turn is determined by the phase relation between thegrid voltage (.3 and anode voltage a. It will be understood that whilethe system lends itself readily to a continu ous' change in outputcurrent for a correspondingly continuous change in the reactive.constants of the bridge, the. system is equally applicable to abruptchanges in the constants as when the photo-electric'tub'e is subjectedfirst to a darkened state and then suddenly to an illuminated conditionor vice versa'.

According to another feature of'mv invention, I have discovered that inaddition to serving as a means for neutralizing the capac-- itances C0;; existing between the electrodes of a discharge tube andphoto-electric tube thereby to obtain maximum sensitivity of optheillumination of the photo-electric tube.

These condenser adjustments consist in producing an over-neutralizationof the interelectrode capacitances C C and it has been found that with acertain degree of neutralization, depending upon the characteristics ofthe discharge tube, photo-electric tube and associated circuits, therelation between output current and the illumination becomes nearlylinear or directly proportional. When adjusted in this manner, it isapparent the circuit has utility in many applications' where a uniformproportionalityis required. For example, it may be necessary to cut outresistance in a lamp load circuit by a device responsive to the tube outut current wherein the device is to be actuated in a progressive mannerprecisely in accordance with changes of the intensity of light impingingon the photo-electric tube. Fig. 4: shows three curves plotted againstoutput current and photo-electric tube illumination under conditions ofover neutralization of the in ter-electrode capacity. It will be notedthat curve cl which was taken under conditions of havingthe capacitanceC adjusted in a given circuit to give ten times the amount of inter- 95electrode capacitance (C C is substantially,

a straight line, indicating an approximate linear relation between.output current and photo-electric tube illumination. On the other hand,curve 6 shows the relation be- 0 tween these factors when C is twice (CC8) in the same circuit as before while curve 7 is predicated on anadjustment in which 0 only slightly over-neutralizes the inter-electrodecapacity. It will be evident that curves e and 1 represent a much moresensitive adjustment than curve (1 but lack the strict proportionalityfeature. Moreover, the curves also show that as the value of Capproaches that of (Cd-C the degree of sensitivity in v the-system isincreased assuming the operation to take place on the steep portion ofthe characteristic. The best value for the capacitance C depends uponthe type of reduced into the system of Figure 1 under different degreesof photo-electric. tube illumi' nation when the condenser 7 is set to(a) under-neutralize b) exactly neutralize and (c) "over-neutralizerespectively, the capacitances (1 C In these diagrams, the linesmarked1, 2, 3 and 4 represent the voltage applied to thegrid under differentdegrees of @1125 illumination, line 1 being typical of the weakestlight. It will be observed that in the case of under-neutralization andbalanced conditions (Diagrams G and H), the grid voltage is morepositive than the critical starting voltage under all conditions ofillumination and hence, current will flow in the discharge tubethroughout the entire positive half-cycle or substantially so, asindicated by the area in cross-hatch. On the other hand, when the.condenser 7 is sufficiently large to over-neutralize the inter-electrodecapacitances (C l-C a condition depicted by the curves in Diagram Iresults. It will be noted that the efi ect of different degrees ofillumination when the system .is over-neutralized is to move theposition of intersection between the grid voltage curves 1, 2, 3 and 4and the critical voltage curve I) so that the starting of the arc occursearlier in the cycle, as the light intensity is increased.

The total current per cycle for the weak light represented by curve 2 isshown by the cross hatched area. Similar areas may be drawn for othervalues of light corresponding to curves 3 and 4 and it is evident thatthe total current per cycle will increase progressively with increasedillumination.

While I have explained my invention with particular reference to thecombination of an arc discharge device and a photo-electric tube, itwill be understood that modifications of the apparatus may be readilymade within the principles of the invention. As examples of suchchanges, it will be evident that a selenium cell may be substituted forthe photo-electric tube and that many of the advantimes of the inventionwould be present in case a high vacuum tube were substituted for the arcdischarge device. Furthermore, the use of the neutralizing condenser 7is not limited to systems employing are discharge devices but also hasapplication in photo-electrictube-amplifier combinations employing highvacuum tubes when energized by alternating voltage. V

In the case of high vacuum tubes with a direct current voltage supply,the neutralizing condenser 7 is not needed. With arc discharge deviceson the other hand, employed in conjunction with a direct current supplyvoltage, the arc will frequently start when the anode switch is closedeven though the grid-is biased at a sufficiently negative potential toprevent starting under constant voltage conditions. This is due to thecapacitycoupling between the grid andplate which makes the gridinstantaneously positive when the plate suddenly becomes positive. Thisimpulsive starting might be minimized by closing the anode circuit firstthrough a very high resistance so that it attains its positive voltageslowly, and then short-circuiting this resistance. It may be entirelyavoided however. bv the use of the neutralizing condenser 7 connectedbetween the grid and a point of opposite surge polarity to that of theanode. One method of obtaining this result is to use the circuit of Fig.1, replacing the transformer 5 by a resistance energized by directcurrent. The difference between the two types of tubes with a directcurrent supply as regards the use of a neutralizing condenser, is thatin the case of a high vacuum device the plate current flows only whilethe grid is positive, which is of the order of a microsecond so that theamount of such current is negligible. In the case of the arc dischargedevice however, a positive grid potential last ing only a micro-secondis sufficient to start the current which will continue thereafter untilthe anode circuit is opened.

With alternating current power, in the event it is desired to turn offthe current in response to the illuminationof the light sensitivemember, the condenser 7 and the light sensitive member would occupy aposition reverse to that shown in Fig. 1, i. e. the condenser 7 in thefigure would be replaced by the said member and the photo-electric tubeby the condenser. In this case. current loes not decrease continuouslywith increasing illumination but falls discontinuously from full valueto zero when the illumination reaches a critical intensity. Thelight-sensitive member in this case should be a twoway resistance, forexample, a selenium cell or a photo-electric tube having two platescoated with a light-sensitive material. If a one-way resistance beemployed for this purpose,'e. g. a photo-electric tube of ordinaryconstruction which conducts current during only one-half of the cycle,the current will be turned oflt' in response to the light but will notstart again when the illumination is removed.

It will also occur to those skilled in the art that other sources ofcontrolling impulses may be utilized in place of the photo-electrictube. For example, a signal which it is desired to amplify may beimpressed between the grid 3 and the anode 4, and if of sufiicientmagnitude to unbalance the bridge, conduction of current will take placebetween the cathode 2 and anode 4 through the load circuit. In'all ofthese cases, the condenser 7 is necessary to ofi-set the grid-anodecapacitance of the arc discharge device for otherwise the latter willstart and continue to operate even with no signal, exactly as explainedhereinbefore in connection with the photo-electric tube in the case ofno illumina- While I have described and illustrated several preferredembodiments of my invention, I donot wish to be restricted thereto, butI desire that only such restrictions shall be placed upon my inventionas are set forth in the appended claims.

What I claim as new and desire to sect...

member and an anode, .a source of alternating current connected betweenthe cathode and anode and serving as the source of current supply forthe cathode-anode circuit, and means connected to said source forneutralizing thegrid-plate capacity efiect ofsaid- "taining a "cathode,an electrostatic control member and an anode, a source of alternatin gcurrent connected between the cathode and anode and serving asthe sourceof current supply for the cathode-anode circuit, a condenser connectedbetween the grid"and a point on said source whose voltage phase withrespect to the cathode is o posite to that of the point to which the anoe is connected.

5. In combination, a thermionic device containing a cathode,anelectrostatic control member and an anode, a source of alternatingcurrent connected between the cathode and anode and serving as thesource of ourrent supply for the cathode-anode circuit, and a condenserconnection between the control member and a point on said source of oposite phase to the anode, the electromotive orce at said point beingsuch that the capacity action between the 1d and plate is neutralized.

6, In combination, a thermionic device containing a cathode, anelectrostatic control member and an anode, a source of alternatingcurrent connected between the cathode and anode, a condenser connectedbetween the grid and a point on sa id source whose voltage phase withrespect to the cathode is opposite to (that of thepoint .to which theanode is connected and a photo-electric tube connected between K thecontrol "member and said source.

7 In combination a discharge tube. comprisin an evacuated envelopecontaining a thermionic cathode, an electrostatic control member,an-anode and an inertgas at a pressure sufiicient to sup ort an arc-likedischarge at the impressed vo tages, a source of alternatingcurrentconnected between the cathode and anode, a condenser connectedbetween the grid and a point on'said source whose voltage phase withrespect to the cathode is opposite to that of the point to which theanode is connected 8.- In combination a discharge tube comprisinganevacuated' envelope containing a thermlonic cathode, an electrostaticcontrol member, an anode and a small amount of inert gas, a source ofalternating current connected between the cathode and anode, a condenserconnected between the grid and a 'point'on said source whose volta ephase with respect to the cathode is opposite to that of the point towhich the anode is connected, the electromotive force at said pointbeing such as to over-neutralize the capacity action between the gridand plate.

' 9. In combination, a discharge tube comprising an evacuated envelopecontaining a thermlonic cathode, an electrostaticcontrol member, an,anode and a small amount of inert gas, a source of alternating currentconnected between the cathode and anode, a condenser connected betweenthe grid and a point on said source whose voltage phase with respect tothe "cathode is oppositeto that of the point to which the anode isconnected, the electromotive force at said point being such as toover-neutralize the capacity action between the grid and plate, and aphoto-electric tube having a cathode connected to said control memberand an anode connected to a point onsaid source of the same phase as theanode of said discharge tube.

10. In combination, a discharge tube comprising an evacuated envelopecontaining a thermionic cathode, an electrostatic control member, ananode and an inert as at a pres 11. In combination, a discharge tubecomprising an evacuated envelope containing a thermionic cathode, anelectrostatic control member, an anode and a small amount of inert gas,a source of alternating current associated therewith, and means forneutralizing the effect of the capacitance between the control memberand the anode, and a photoinemberand said source.

12. In comblnation, a thermiomc device comprising an evacuated envelopecontaining .a cathode, an electrostatic control member, an

anode and-an inert gas at a pressure suflicient to support an arc-likedischarge at the impressed voltages, a source of alternating current forenergizing said device, a photoelectric tube connected between? saidcontrol electric tube connected between the control member and source, aload circuit connected between the cathode and anode.

13. A system for amplifying photo-elec tric currents comprising anelectric bridge arrangement having a plurality'of inter-connected arms,and amplifier connected to the bridge, said amplifier including acathode, an anode and an electrostatic control member, one of the armsof the bridge comprising a capacitance, said arm being Joined to anotherarm which includes a photo-electric tube, the bridge being closed by asource of energy, electrlcal connections between the control member andthe junction of the arms containing the capacitance and photo-electrictube, also between the cathode and an intermediate tap on said source ofenergy, and also between the anode and the junction of the armscontaining the photo-electric tube and a portion of the source ofenergy.

14. In combination, a photo-electric tube,

means for amplifying the photo-currents,

said means comprising an envelope containing a cathode, a grid, an anodeand an inert gas at a ressure sufiicient to support an arclike disc argeat the impressed voltages, a source of alternating current forenergizing the photo-electric tube and the amplifier, saidphoto-electric tube being connected between the id and a terminal ofsaid source and a con enser connected between said grid and a terminalof said source, which is of opposite polarity to said first mentionedterminal whereby a variable activation of the photoelectric tube servesto shift the phase of the grid voltage with respect to voltage appliedto the anode thereby to initiate an are discharge between the said anodeand cathode in response to changes in activation of the photo-electrictube.

15. The method of operating a light-responsive apparatus which consistsin subjecting the electrostatic control vmember of a three-electrodedevicesubstantially, to equal and op osite potential impulses wherebysubstantia 1y no. space current flows between the anode and cathode ofsaid device, and then shifting the voltage phase ofone im ulse withrespect to that of the other in accor ance with variations of lightwhereby current is initiated in the anode-cathode circuit which variesin responseto the variations in light.

In witness whereof, I have hereunto set my hand this 22nd da of June,1929.

AJZBERT HULL.

